High voltage stabilizing circuit

ABSTRACT

A high voltage stabilizing circuit comprises a high voltage generating circuit (FBT, D1) for boosting a pulse voltage (Vcp) generated from a horizontal deflection circuit (Q1, D2, Ct&#34;, Ly, Cs) during a flyback period of a horizontal deflection pulse, a flyback period control circuit (Q2, C B , Rv, Ct&#39;) for performing switching operation only during the flyback period to control the flyback period, so that the high voltage (HV) is controlled, and a switching control resistor (Rv) for controlling the period of its switching operation.

TECHNICAL FIELD

The present invention relates to a high voltage stabilizing circuit welladaptable for a monitoring apparatus using a cathode ray tube, atelevision receiver, a display device or the like.

BACKGROUND ART

In a conventional monitoring apparatus in which the removal of a highvoltage variation is important, a high voltage generating circuit and ahorizontal deflection circuit are provided separately and a stabilizingcircuit is also provided in the high voltage generating circuit,although these arrangements are complicated. When considering costreduction and power economy, however, a high voltage circuit of aso-called flyback transformer type in which a high voltage is generatedby a flyback transformer serving both as the high voltage generatingcircuit and the horizontal deflection circuit, is superior to the abovecircuit arrangement.

FIG. 1 shows an arrangement of a conventional high voltage circuit ofthe flyback transformer type, which takes no measure for voltagestabilizing. In the circuit, when a drive pulse is applied to the baseof a horizontal output transistor Q1, it produces a horizontaldeflection pulse. During the flyback period of the horizontal deflectionpulse, a collector pulse voltage Vcp is produced. After the voltage Vcpis boosted to a high voltage by a flyback transformer FBT, the boostedvoltage is further rectified by a high voltage rectifying diode D1 toobtain an output of a DC voltage HV. Here, D2 designates a dumper diode,Ct a resonating capacitor, Ly a horizontal deflection coil, and Cs a DCblocking capacitor. A DC voltage Vcc is supplied to the terminal of theprimary side of the flyback transformer FBT.

The collector pulse voltage Vcp is generally expressed by ##EQU1## whereT_(H) is the duration of one period of a horizontal deflectionfrequency, and T_(R) is a flyback period.

When a load of the high voltage circuit is fixed, the DC high voltage HVis proportional to the collector pulse voltage Vcp. Accordingly, whenthe high voltage load varies and the DC high voltage HV decreases, ifthe flyback period T_(R) in equation (1) is reduced, followingproportionally the decrease of the DC high voltage HV, the collectorpulse voltage Vcp increases and the DC high voltage HV also rises.

The flyback period T_(R) is generally expressed by the followingequation: ##EQU2## where Lyi is an inductance of the deflection coil Ly,and Ctc is a capacitance of a resonating capacitor Ct.

As seen from the above equations (1) and (2), if the inductance Lyi ofthe deflection coil Ly and/or the capacitance Ctc of the resonatingcapacitor Ctc is changed according to a variation of the high voltageload, the DC high voltage HV can be stabilized.

A conventional high voltage stabilizing circuit of the reactor type inwhich the inductance Lyi is changed has many disadvantages includingpoor transient response, heavy weight, large size, high manufacturingcost and so on. To overcome those disadvantages, another high voltagestabilizing circuit which controls the collector pulse voltage Vcp bychanging a capacitance Ctc of the resonating capacitor Ct, is disclosedin Japanese Patent Application Laid-open No. 56-134879. In such aconventional stabilizing circuit, however, the control operation iscontinuously performed during periods other than the flyback periodT_(R). As a result, the circuit loss is large, and hence the powereconomy is not attained. Additionally, the circuit operation is notstable due to heat generation in the circuit.

DISCLOSURE OF THE INVENTION

Accordingly, with the view of removing the above-mentioneddisadvantages, the present invention has an object to provide a highvoltage stabilizing circuit in which the stabilization of the DC voltageand the power saving are both realized and the instability of thecircuit operation due to the heating of the circuit is also removed.

To achieve the above object, in the present invention, a flyback periodcontrol circuit is operated only during the flyback period of thehorizontal deflection pulse to control the flyback period according to aload variation of high voltage.

To be more specific, in the present invention, a high voltagestabilizing circuit has a high voltage generating means for boosting apulse voltage produced during a flyback period of a horizontaldeflection pulse produced from a horizontal deflection circuit, aflyback period control means for controlling the high voltage byperforming switching operation only during the flyback period to controlthe flyback period, and a switching control means for controlling aperiod of the switching operation.

The flyback period control means may include a control transistor, acapacitor for differentiation connected between a first electrode(collector) and a control electrode (base) of a control transistor, anda resistor, for example, a variable resistor, connected between thecontrol electrode (base) of the control transistor and a secondelectrode (emitter), and the pulse voltage is applied to the firstelectrode (collector) of the control transistor.

A diode may be connected between the control electrode (base) and thesecond electrode (emitter) of the control transistor.

To the first electrode (collector) of the control transistor, the pulsevoltage may be supplied through a capacitor.

The switching control means may include a voltage dividing means fordividing the high voltage to have a divided voltage, and an errordetecting means for comparing the divided voltage with a predeterminedreference voltage to obtain an error voltage.

The voltage dividing means may include a resistor for dividing the highvoltage and an amplifying circuit for amplifying the divided voltage.

The error detecting means may include a transistor to which the dividedvoltage or a voltage obtained by amplifying the divided voltage and thereference voltage are applied, so that an error voltage is derived fromthe transistor, and the error voltage is then applied to the flybackperiod control means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an arrangement of a conventionalhigh voltage circuit of the flyback transformer type;

FIG. 2 is a circuit diagram showing an embodiment of an arrangement of ahigh voltage stabilizing circuit according to the present invention;

FIG. 3 is a circuit diagram showing an equivalent circuit of the circuitshown in FIG. 2;

FIGS. 4A-4G are signal waveforms illustrating one example of currentsand voltages at various portions in a flyback period control circuitshown in FIG. 2; and

FIG. 5 is a circuit diagram showing another embodiment of an arrangementof a high voltage stabilizing circuit according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail with reference to theaccompanying drawings.

FIG. 2 shows an embodiment of an arrangement of a high voltagestabilizing circuit according to the present invention. In the figure,the same reference numerals as in FIG. 1 are used for designating likeportions in FIG. 1, and no further explanation thereof will be given. InFIG. 2, reference numeral 1 designates a flyback period control circuitconnected between the primary coil of the flyback transformer FBT andthe collector of the horizontal output transistor Q1. As will bedescribed later, only during the flyback period T_(R) of a deflectioncurrent flowing into the horizontal deflection coil Ly does the controlcircuit 1 perform the switching operation to control the flyback periodT_(R). The flyback period control circuit 1 has a control transistor Q2which performs the switching operation only during the flyback periodT_(R) by a control signal supplied to the base thereof, a differentialcapacitor C_(B) connected between the collector and the base of thetransistor Q2, a variable resistor Rv connected between the base of thetransistor Q2 and the common potential and for setting a voltage to beapplied to the base thereof, a protecting diode D3 connected in parallelwith the variable resistor Rv between the base of the transistor Q2 andthe common potential, and a resonating capacitor Ct' connected betweenthe collector of the transistor Q1 and the collector of the transistorQ2. As shown, in the present embodiment, the resonating capacitor Ctshown in FIG. 1 is composed of the capacitors Ct' and Ct".

The operation of the circuit shown in FIG. 2 will be described withreferring to FIGS. 3, and 4A-4G. The circuit shown in FIG. 3 is anequivalent circuit depicted with neglecting the internal resistance ofthe transistor Q2 in FIG. 2, in which Cob is a capacitor between thecollector and the base of the transistor Q2. FIGS. 4A-4G illustratewaveforms of currents and voltages at respective portions in the flybackperiod control circuit 1.

The current flowing into the variable resistor Rv is a resonatingcurrent i_(r) supplied through the deflection coil Ly and the resonatingcapacitors Ct' and Ct", as shown in FIG. 3, and hence the current i_(r)has a waveform as shown in FIG. 4B. A waveform of a voltage across thevariable resistor Rv, i.e. a voltage VBE between the base and theemitter of the transistor Q2, is also substantially identical to that ofthe current i_(r) (see FIG. 4E). The transistor Q2 is conductive whenthe base potential V_(BE) of the transistor Q2 reaches the forward biaspotential V_(BES) in a slanted portion in FIG. 4E (see FIG. 4G).

In the negative portion of the waveform of the resonating current i_(r)shown in FIG. 4B, the transistor Q2 is biased backward and when its basevoltage V_(BE) reaches V_(BER), a backward current flows from the baseto the collector of the transistor Q2 (see FIGS. 4C and 4F). At thistime, the base potential of the transistor Q2 is clamped by the forwardvoltage of the diode D3 in order to protect the transistor Q2 from beingbroken (see FIG. 4D). In the figure, ic denotes the collector current ofthe transistor Q2, Vcp' the voltage between the collector and theemitter of the transistor Q2, i_(B) the base current of the transistorQ2, and i_(E) an emitter current of the transistor Q2.

Accordingly, the transistor Q2 performs the switching operationaccording to a change of the voltage V_(BE) between the base and theemitter of the transistor Q2, and is conductive during a period T1 whenit is forward-biased and a period T2 when it is backward-biased, and isinterrupted during a period T3 between the periods T1 and T2.

If a resistance of the variable resistor Rv is changed, the voltageV_(BE) across the resistor Rv changes and accordingly the conductionperiods T1 and T2 change. In this manner, the switching timing of thetransistor Q2 can be changed. For example, if the resistance of thevariable resistor Rv is made zero, the bias current i_(B) is notsupplied to the transistor Q2, so that the resonance capacitance is thesum of the capacitances of the series-connected capacitors Ct' andC_(B). Accordingly, the flyback connected capacitors Ct' and C_(B).Accordingly, the flyback period T_(R) is shortened and the DC highvoltage HV rises.

On the other hand, when the resistance of the variable resistor Rv isincreased and a forward bias voltage is applied to the transistor Q2,the transistor Q2 can be made conductive substantially during the entireflyback period T_(R) . The resonating capacitance increases, since thecapacitors Ct' and Ct" are rendered to a condition that the capacitorCt' is coupled in parallel to the capacitor Ct". Accordingly, theflyback period T_(R) is extended and the DC high voltage HV drops (seeequations (1) and (2)). Therefore, by changing the resistance of thevariable resistor Rv according to a variation of the DC high voltage HV,the DC high voltage HV can be stabilized.

It is desirable, from a viewpoint of the high voltage stability, thatthe flyback period control circuit 1 is used in the form of a closedloop. Thus, another embodiment of a high voltage stabilizing circuitaccording to the present invention, which is designed so as to realizesuch a desirable arrangement, is shown in FIG. 5. In the figure, thesame reference numerals as in FIG. 2 are used to designate like portionsin FIG. 2. In the circuit, R1 and R2 represent, respectively, voltagedividing resistors for dividing the DC high voltage HV obtained from thehigh voltage side (secondary side) of the flyback transformer FBT, andVR1 a voltage dividing variable resistor for dividing the DC highvoltage HV. These resistors R1, R2 and VR1 are connected in series andthe resistor R1 has one terminal which is grounded. One terminal of theresistor R2 is connected to the secondary terminal of the flybacktransformer FBT, through the high voltage rectifying diode D1.

A capacitor C1 and a resistor R3 are connected in series between thegate and the drain of a field effect transistor Q3, and a resistor R4 isconnected between the drain thereof and the common potential. Thecollector of a transistor Q4 and the emitter of the transistor Q2 areconnected to each other, and the emitter of a transistor Q4 and the baseof the transistor Q2 are also connected to each other. A main powersource, e.g. 110 V, of a monitoring apparatus or the like is applied asa reference voltage V_(REF) to the collector of the transistor Q4.Connected between the collector of the transistor Q4 and the gate of thefield effect transistor Q3 is a protecting diode D4 for preventing theapplication of an over-voltage.

A detected voltage formed by dividing the DC high voltage HV by theresistor R1 and the variable resistor VR1 is determined to be about1/250 of the DC high voltage HV when the voltage HV is set, for example,at 27.5 KV. The detected voltage is applied to the gate of the fieldeffect transistor Q3, through the sliding terminal of the variableresistor VR1. By changing the variable resistor VR1, the detectedvoltage can be adjusted at a desired value. The field effect transistorQ3 amplifies the detected voltage up to a given current level. Theamplified output current is derived from the source of the field effecttransistor Q3, and is supplied to the base of the error amplifyingtransistor Q4 connected to the source of the field effect transistor Q3.

An impedance between the collector and the emitter of the transistor Q4changes in response to a change of the detected voltage applied to thebase thereof, and therefore the transistor Q4 behaves like a variableresistance element corresponding to the variable resistor Rv shown inFIG. 2.

The reference voltage V_(REF) is applied to the emitter of thetransistor Q2 as well as the collector of the transistor Q4. Adifferential resistor R5 is connected between the base and the emitterof the transistor Q2, and an electrolytic capacitor C2 is connectedbetween the emitter thereof and the common potential. To the collectorof the horizontal transistor Q1, a DC voltage Vcc having the samepotential as that of the reference voltage V_(REF) is applied by way ofthe primary winding of the flyback transformer. Further, L_(IN)designates a coil for deflection linearity correction, D5 a diode, andC3 a capacitor.

The operation of the circuit shown in FIG. 5 will now be explained.

A horizontal deflection drive pulse is applied to the horizontaltransistor Q1 so that the transistor Q1 operates as a horizontal outputcircuit. Under this condition, during its flyback period T_(R), a largecollector pulse voltage Vcp is generated. The voltage Vcp is boosted bythe flyback transformer FBT to generate the DC high voltage HV. In thiscase, the resonating current i_(r) flows through the capacitors Ct' andC_(B), and further into the resistor R5, so that a voltage V_(BE) isproduced between the base and the emitter of the transistor Q2. Thewaveform of the voltage V_(BE) is substantially the same as that of theresonating current i_(r), as described above (see FIGS. 4B and 4E).

Then, if the DC high voltage HV drops due to a variation of the load,the detected voltage obtained from the voltage division by the voltagedividing resistor R1 and the variable resistor VR1 drops, so that thedrain current of the transistor Q3 increases. The current is supplied tothe base of the transistor Q4 and an impedance between the collector andthe emitter of the transistor Q4 decreases. The decrease of theimpedance of the transistor Q4 shortens the conduction period of thecollector current of the transistor Q2. As a result, an apparentcapacitance of the capacitor Ct' decreases and the resonatingcapacitance reduces. Therefore, the collector pulse voltage Vcpincreases and accordingly an average value of the DC high voltage HVincreases, so that the DC high voltage HV is stabilized.

On the other hand, when the DC high voltage HV rises, the conductionperiods T1 and T2 of the transistor Q2 are extended, while theinterruption period T3 shown in FIG. 4E is relatively shortened.Accordingly, the resonating capacitance increases, and the flybackperiod TR increases. As a result, the collector pulse voltage Vcp drops,so that the DC high voltage HV is stabilized.

As seen from the foregoing description, the present embodiment employs afeedback control system in which a variation value of the DC highvoltage HV is fed back to the control circuit to cancel the variation.Therefore, its operation stability is high. If the main power source ofthe monitoring apparatus or the like is directly used for the referencevoltage V_(REF) in the high voltage feedback, the provision of anadditional reference voltage source is not required.

POSSIBLE APPLICATIONS IN THE INDUSTRY

As described above, according to the present invention, the switchingcontrol is performed only during the flyback period of the horizontaldeflection pulse to stabilize the DC high voltage, so that the highvoltage stabilizing circuit according to the present invention issuitable for saving power consumption, and can solve the problem thatthe circuit does not operate stably due to the heating of the circuit.

I claim:
 1. A high voltage stabilizing circuit, comprising:high voltagegenerating means: for boosting a pulse voltage produced from ahorizontal deflection circuit during a flyback period of a horizontaldeflection pulse to obtain a high voltage; operations period controlmeans for performing switching operations, only during said flybackperiod, to controls aid flyback period, the flyback period control meansincluding a variable resistor for changing the resistance according tovariations of the high voltage, thereby controlling said high voltage;and switching control means for controlling a period of said switchingoperation.
 2. A high voltage stabilizing circuit according to claim 1,wherein said flyback period control means includes a control transistorhaving base, emitter and collector electrodes, a differential capacitorconnected between the collector electrode of said control transistor andthe electrode of said control transistor, and the variable resistorconnected between said base electrode of said control transistor and acommon potential, and said pulse voltage is applied to said baseelectrode of said control transistor.
 3. A high voltage stabilizingcircuit according to claim 2, wherein a diode is connected between saidbase electrode and said common potential.
 4. A high voltage stabilizingcircuit according to claim 2 or 3, wherein said pulse voltage issupplied to said base electrode of said control transistor through acapacitor.
 5. A high voltage stabilizing circuit according to claim 4wherein said switching control means includes voltage dividing means fordividing said high voltage to obtain a divided voltage, and errordetecting means for comparing said divided voltage with a predeterminedreference voltage to obtain an error voltage.
 6. A high voltagestabilizing circuit according to claim 5, wherein said voltage dividingmeans includes resistors for dividing said high voltage and anamplifying circuit for amplifying the divided voltage.
 7. A high voltagestabilizing circuit according to claim 6, wherein said error detectingmeans includes a transistor to which said divided voltage or a voltageobtained by amplifying said divided voltage and said reference voltageare supplied, and an error voltage is derived from said transistor andis then supplied to said flyback period control means.
 8. A high voltagestabilizing circuit as set forth in claim 1 wherein if the variableresistance is made 0, the flyback period is shortened and the highvoltage rises.
 9. A high voltage stabilizing circuit as set forth inclaim 8 wherein when the resistance of the variable resistor increases,the flyback period is extended and the output voltage drops.
 10. A highvoltage regulation circuit for use in a high voltage power supplycircuit, the power supply circuit for supplying a high voltage to acathode ray tube, the power supply circuit having a flyback transformer,a horizontal deflection circuit connected to a primary winding of saidflyback transformer for supplying flyback pulses to the primary winding,and a step up rectifying circuit for rectifying the stepped up flybackpulses to obtain said high voltage which is supplied to said cathode raytube, said regulation circuit comprising:a control transistor having anemitter connected to ground or a low impedance power source; a firstcapacitor connected between said primary winding of said flybacktransformer and a collector of said control transistor; a secondcapacitor connected between said collector and a base of said controltransistor; a diode connected between said base and said emitter of saidcontrol transistor; and a variable conductivity device connected betweensaid base and said emitter of said control transistor, said variableconductivity device being driven in response to the change of said highvoltage in a manner that the conductivity of said device increases whensaid high voltage decreases, thereby performing switching operation, theperiod of which is changed by said variable conductivity device, thevariable conductivity device performing switching operation only duringa flyback period of a horizontal deflection pulse to control saidflyback period to control said high voltage.
 11. The high voltageregulation circuit as claimed in claim 10, wherein said variableconductivity device is a variable transistor.
 12. The high voltageregulation circuit as claimed in claim 10, further comprising:voltagedividing means for dividing said high voltage to obtain a dividedvoltage; and wherein said variable conductivity device includes errordetecting means for comparing said divided voltage with a predeterminedreference voltage to obtain an error voltage for controlling saidcontrol transistor.
 13. The high voltage regulation circuit as claimedin claim 12, wherein said voltage dividing means includes resistors fordividing said high voltage and an amplifying circuit for amplifying thedivided voltage.
 14. The high voltage regulation circuit as claimed inclaim 12, wherein said error detecting means includes a transistor towhich said divided voltage and said reference voltage are supplied, sothat an error voltage is derived from said transistor and is thensupplied to said control transistor.